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type-error

type-error Condition Type

Class Precedence List:

type-error, error, serious-condition, condition, t

Description:

The type type-error represents a situation in which an object is not of the expected type. The “offending datum” and “expected type” are initialized by the initialization arguments named :datum and :expected-type to make-condition, and are accessed by the functions type-error-datum and type-error-expected-type.

See Also:

type-error-datum, type-error-expected-type

Expanded Reference: type-error​

Handling a type-error​

A type-error is signaled when an object is not of the expected type. You can catch it with handler-case.

(let ((x nil))
  (handler-case
      (check-type x integer)
    (type-error (c)
      (format nil "Got type-error: datum=~S, expected=~S"
              (type-error-datum c)
              (type-error-expected-type c)))))
=> "Got type-error: datum=NIL, expected=INTEGER"

Signaling a type-error Manually​

(defun positive-integer-p (x)
  (unless (and (integerp x) (plusp x))
    (error 'type-error :datum x :expected-type '(integer 1))))
=> POSITIVE-INTEGER-P

(handler-case (positive-integer-p -5)
  (type-error (c)
    (type-error-datum c)))
=> -5

type-error in the Wild​

Many standard functions signal type-error when given inappropriate arguments.

(handler-case (+ "a" 1)
  (type-error (c)
    (format nil "Expected ~A, got ~S"
            (type-error-expected-type c)
            (type-error-datum c))))
=> "Expected NUMBER, got \"a\""